I have never heard of frozen helium.
To create a solid, a compound has to be able to generate inter- atomic (or inter-molecular) forces
to bind the atoms together into a lattice.

For helium, the inter-atomic forces are tiny,
and also they have no asymmetry so there is no preferred direction for the forces
so there is no "guidance" toward a lattice structure.
I guess it could create a "glass" (a disordered structure) rather than a crystal (ordered structure)

Having said all that, I suspect that as the temperatures approach absolute zero,
the behaviours of materials start to go beyond the simple everyday solid/liquid descriptions.
It is possible that you start to enter the world of quantum condensates before you get to solid helium.

There is a strange philosophical issue about an object at absolute zero.
How would you know it existed?

If you illuminated it, or touched it, or in fact did anything to verify that it was in fact there,
you would unavoidably heat it up!

Maybe just a tiny fraction, but then it would not be at absolute zero any more.

Yep. The only thing I can think of is orbital electron photoabsorption and reemission, which is fully reversible, but if you do that, the behaviour of the state you're probing is now the excited state, which is definitely not absolute zero, even if the initial and final temperatures were both at absolute zero.

Yep. The only thing I can think of is orbital electron photoabsorption and reemission, which is fully reversible, but if you do that, the behaviour of the state you're probing is now the excited state, which is definitely not absolute zero, even if the initial and final temperatures were both at absolute zero.

How is that if the orbital electron was in its ground state, and absorbs a photon raising to a higher energy level, before reemitting the photon. does it not return to its ground state. The total energy of the electron and photon before the absorption and after its emission must the the same. If not and a photon with the same energy is emitted where would the extra energy come from

How is that if the orbital electron was in its ground state, and absorbs a photon raising to a higher energy level, before reemitting the photon. does it not return to its ground state. The total energy of the electron and photon before the absorption and after its emission must the the same. If not and a photon with the same energy is emitted where would the extra energy come from

That assumes that the electron falls back down into the ground state all in one go.

For example, a "semi-stable state" of an electron in an atom is a state that is relatively long lived. Optical pumping in lasers and in some cryogenic systems are a good example of the use of a semi-stable state.

So let's go ahead and talk about 2 excited states for the electron. Call the ground state electron GS, the first excited state FE, and second excited state SE. We are going to introduce a photon with energy SE - GS, which will promote an electron from the ground state to the SE state. This electron can now decay in one of two modes: directly back to the GS or to the FE state. If the FE state is a semi-stable state then the electon emits a photon with an energy of SE - FE. It is the SE - FE energy that is the goal in building lasers, etc.

The diagram is a bit rough. I couldn't find a good graphic for it and this idiotic version of MS Paint is even worse than the old one!